The invention relates to a steering system for motor vehicles with a steering member, in particular a steering wheel, which is driver actuated and is hydraulically positively coupled to steered vehicle wheels. A hydrostatic master unit, which is mechanically drive-connected to the steering member, and a hydrostatic slave unit, which mechanically drive-connected to steered vehicle wheels, are drive-connected to one another via hydraulic lines. A servo system can be controlled as a function of the forces and torques transmitted between the steering member and the master unit.
A known steering system of the aforementioned type is described in DE 41 33 726 A1. In current motor vehicle steering systems, however the steering wheel is mechanically positively coupled to the steered vehicle wheels via a so-called steering column, which requires considerable amount of installation space. Moreover, comparatively complex designs are required in order to reliably avoid a situation, in the case of motor vehicle accidents, where the steering column can push the steering wheel into the vehicle interior against the driver. A further difficulty is that comparatively extensive design modifications are required if a motor vehicle is to be produced with both left-hand and right-hand steering.
The hydraulic drive connection between the steering wheel and the steered vehicle wheels in accordance with DE 41 33 726 A1 eliminates the need for the accommodation of the steering column. The hydraulic lines provided instead can be accommodated in virtually any desired manner.
Despite these apparent advantages, it has not hitherto been possible for steering systems of the type described in above-mentioned DE 41 33 726 A1 to be used in motor vehicles because of lack of legal approval. Doubts about safety are probably the primary reason for this.
Another conventional steering system is described in German Offenlegungsschrift 2,261,144 in which the servo system is integrated into the hydraulic positive coupling.
GB-A 977 193 discloses only a steering system with a hydraulic drive connection between the steering wheel and steered vehicle wheels without a servo unit.
DE 37 14 833 A1 relates to a servo steering system with a mechanical drive connection between the steering wheel and steered vehicle wheels and two servo systems arranged in parallel. If a normally active hydraulic servo valve becomes inoperative, an electric servo motor can come into action.
It is, therefore, an object of the present invention to ensure particularly high reliability in a steering system.
According to the present invention, this object has been achieved by loading the master unit into a central position by springs.
The present invention is based on the recognition that, when the master unit is adjusted by the steering member, a manual force of greater or lesser magnitude must be overcome and is provided by virtually indestructible and fail-safe springs. By controlling the servo system, which is independent of the hydraulic drive connection between the steering member and the steered vehicle wheels, as a function of the forces acting between the steering member and the master unit, a servo force suitable for the actuation of the steered vehicle wheels can be produced and controlled even if the hydraulic drive connection fails completely due to leakage.
Given a functional hydraulic drive transmission, the servo system of the present invention thus serves to increase comfort, while, if the hydraulic drive transmission is interrupted, maintaining the steerability of the vehicle.
The hydraulic connecting lines can be dimensioned so as to have a restrictor effect or can be fitted with throttle valves in order to achieve the action of a steering damper without further measures
According to a particularly preferred embodiment, the hydraulic system can be held continuously under a hydraulic preload. On one hand, complete freedom from play is ensured. On the other hand, the system can be monitored for any pressure drop. A warning signal can be generated as soon as the hydraulic excess pressure drops below a threshold at which reliable operation of the steering is assured for a considerable time to come.